The present invention relates generally to a method, apparatus, and system for synchronization in radio communication systems. In particular, the present invention relates to a method, apparatus, and system for synchronization in radio communication systems using a variably robust information stream.
Over the past decades, advancements in radio and VLSI technology has fostered widespread use of radio communication in consumer electronics. Portable devices, such as mobile telephones, are now widely available offering acceptable cost, size, and power consumption levels.
The first mobile telephones available for public use were analog telephones. These first generation telephones used various analog-based access technologies, e.g., AMPS, NMT, and TACS, to exchange information within a communication region. Consumer interest in mobile radio communication increased dramatically in the 1990's with the introduction of new digital mobile telephones. These second generation telephones used more robust, more secure, and faster digital access technologies, e.g., GSM, D-AMPS, and PDC, to exchange both voice and data information. Although consumer use of radio technology is predominantly in the area of voice communication (at least with respect to handheld devices), the wireless exchange of data is likely to greatly expand in the near future as a result of further technological advances.
Synchronization of the information stream between radio devices is of great importance in any radio communication system. In order to properly recover the information stream sent by a transmitting device, a receiving device must synchronize to the incoming communication signals.
For example, with the so-called continuous wave (CW) radio systems, e.g., first generation analog cordless phones, and the more modern direct-sequence CDMA systems, synchronization of the information stream occurs once at the establishment of a connection. In CW radio systems, a radio receiver initially synchronizes to the received transmission from a radio transmitter signal upon connection establishment. Thereafter, a tracking mechanism is used to maintain synchronization between the radio receiver and transmitter.
In contrast to CW radio systems, so-called packet or burst based radio systems, transmit information between devices in short bursts. In packet-based systems, synchronization is required upon the receipt of each information burst. An efficient synchronization method is therefore essential, in order to minimize the amount of overhead that must be included in the burst to achieve synchronization. The demodulation of the received information stream will be significantly impaired by errors until the synchronization process has been finalized. This will be true even under best transmission conditions, e.g., when the signal-to-noise ratio of the received signal is quite high.
Synchronization is required in radio communication systems because of modulation frequency and timing offsets that exist between the transmitter and receiver devices. Frequency offsets may occur in the received signal as a result of differences in the local oscillator frequencies of the transmitter and receiver devices. These frequency offsets may cause the received signal to not be centered within the band-pass of the receive filters, and may further result in rotating constellations and accumulating phase errors in the demodulated signal. Timing offsets can cause the received symbols to be sampled at sub-optimal sampling intervals, e.g., at sampling intervals where the received symbols are more susceptible to noise and interference, resulting in sampling errors.
Synchronization schemes may be divided into two broad categories. The first category includes the so-called data-aided synchronization schemes. These schemes use known symbol sequences that are inserted within the information stream, e.g. in a packet, to synchronize the data transmission between transmitter and receiver. The known data stream is used to “train” the receiver, that is, to aid the receiver in determining the frequency and timing offsets in the received signal. Hence, these known sequences are often referred to as training sequences.
These training sequences may be inserted at the beginning of the information stream as shown in the packet 102 of FIG. 1. Examples of transmission schemes that organize the information stream in this manner include radio systems based on Bluetooth™, WLAN 802.11, and HIPERLAN2. Alternatively, the training sequences may be inserted in the middle of the information stream as shown in the packet 104 of FIG. 1. This is methodology adopted in the GSM radio system.
Because the information used for the training sequence must be included in the information stream at the expense of the user data, the sequence represents an “overhead” in the communication channel that should be minimized, if at all possible. That is, the number of symbols used to represent the training sequence should be made as small as possible.
The second category of synchronization schemes includes the so-called non-data-aided synchronization schemes. These schemes do not require that any separate, explicit training sequence be included in the information stream for synchronization. As the name suggests, these non-data-aided synchronization schemes use the actual user information stream to train the receiver. Initially, the received information stream may only be used for training the receiver. The stream cannot be immediately demodulated upon receipt because of the errors that would be introduced as a result of the frequency and timing offsets discussed above. Instead, the received information stream must first be stored, and then later demodulated, after the receiver has been fully trained. Thus, the reduction in overhead associated with non-data-aided synchronization schemes comes at the price of increased delay in the demodulation of data and/or increased storage requirements in the receiver.
Not having an explicit training sequence requires that the information streams used to train receivers in non-data-aided synchronization schemes based systems meet certain minimum packet length requirements. That is, the number of symbols in the packet should at least be sufficient for the receiver to train on. Also, the overhead inherent to data-aided synchronization schemes is not completely eliminated with non-data-aided synchronization schemes, as a small frame-delimiter is still required to determine the start of a packet.
One can argue that a form of non-data-aided synchronization is always applied in modern radio communication systems, even when the radio system uses a data-aided scheme of synchronization. This is because most synchronization schemes (both data-aided and non-data-aided) separate the synchronization process into two phases: a coarse phase, and a tracking phase. The second of these two phases, or the tracking phase, does not require that an explicit training sequence exist in the transmitted information stream (i.e., the tracking phase is non-data-aided) in order to maintain synchronization. This is true of the tracking phase whether the coarse phase is data-aided or non-data-aided.
The first of the phases, or the coarse phase, is often alone referred to as “synchronization”. During synchronization, a coarse tuning of the receiver to the received signal takes place. The coarse phase of synchronization has a finite duration during which no demodulation of the received information stream occurs. It is not until a requisite degree of synchronization is established between the receiver and transmitter that a successful demodulation of the information stream may begin.
The coarse and tracking phases of the synchronization process are closely related to one another. When the coarse phase is complete, the receiver then enters the tracking phase of the synchronization process. During the tracking phase, certain receiver parameters are continuously updated to maintain an optimal synchronization with the information stream.
As described above, the tracking phase is non-data-aided, requiring only user information symbols in the stream to estimate the receive parameters needed to maintain synchronization. These parameter estimates are sufficiently accurate to allow the received information symbols to be demodulated at the same time the receiver is being fine tuned to the received signal.
Since information symbols may be demodulated at the same time tracking is applied to the user information stream, the amount of overhead in data-aided synchronization schemes, and the storage requirements and minimum packet length requirements in non-data aided schemes, may be reduced by beginning the tracking phase as quickly as possible in the overall synchronization process.